Recirculating paint system or the like

ABSTRACT

A recirculating pressure system for supplying a plurality of spray guns or the like is disclosed. A single pump supplies pressurized liquid to one end of a single recirculating loop, and the recirculating liquid, after passing through the loop, returns to the pump intake. A plurality of spraying stations are located in the loop and are connected in series. Each spraying station includes an adjustable automatic flow restriction which operates to produce a pressure drop at the associated station. A spray gun is connected to the loop at each station in parallel with the associated flow restriction. The supply line is connected upstream from the flow restriction, and the return line is connected downstream thereof. The flow restrictions function to produce sufficient-pressure drop to insure recirculating flow to the associated gun while permitting a major portion of the liquid to bypass the associated gun.

Levey ]Mareh 13, 1973 1 RECIRCULATING PAINT SYSTEM OR THE LIKE [75]Inventor: Gustave S. Levey, Houston, Tex.

[73] Assignee: GustaveS. Levey, trustee, Houston,

Tex.

221 Filed: Aug.30, 1971 21 App1.No.: 176,226

[52] US. Cl ..239/127 [51] Int. Cl. ..B05b 9/00 [58] Field of Search......239/l24-127 [56] References Cited UNITED STATES PATENTS 3,023,9683/1962 Mitchell ..239/125 2,091,166 8/1937 Shiels i t r ..239/l272,244,686 6/1941 Garrison et a1 ..239/126 2,266,354 12/1941 Christensenet a1. ..239/l26 2,461,766 2/1949 Peeps "239/127 2,727,786 12/1955 Bede..239/l27 3,018,968 l/l962 Levey ..239/l27 Primary Examiner-Lloyd L.King Attorney-Harold F. McNenny et a1.

[57] ABSTRACT A recirculating pressure system for supplying a pluralityof spray guns or the like is disclosed. A single pump suppliespressurized liquid to one end of a single recirculating loop, and therecirculating liquid, after passing through the loop, returns to thepump intake. A plurality of spraying stations are located in the loopand are connected in series. Each spraying station includes anadjustable automatic flow restriction which operates to produce apressure drop at the associated station. A spray gun is connected to theloop at each station in parallel with the associated flow restriction.The supply line is connected upstream from the flow restriction, and thereturn line is connected downstream thereof. The flow restrictionsfunction to produce sufficient-pressure drop to insure recirculatingflow to the associated gun while permitting a major portion of theliquid to bypass the associated gun.

15 Claims, 5 Drawing Figures PATENTEDMAM 3l975 720,373

' sum 1 ur 3 i 24 INVENTOR.

6057 4145 J. zel/gv RECIRCULATING PAINT SYSTEM OR THE LIKE BACKGROUND OFTHE INVENTION This invention relates generally to liquid distributionsystems and more particularly to a novel and improved recirculatingdistribution system for supplying liquid under pressure to a pluralityof airless paint spraying devices or the like.

PRIOR ART Recirculating pressure systems for paint spray devices areknown. Such systemsare normally used when the material being sprayed isheated or when the material tends to settle or separate if continuousflow is not maintained. My prior U.S. Pat. Nos. 3,018,968 and 3,388,866both disclose recirculating systems for supplying a single airless spraygun with a pressurized heated coating material such as paint or thelike. In the system disclosed in the latter of these patents, the paintis pressurized by a pump and is delivered to the spray gun through aheater and a supply line. A return or recirculating line is providedthrough which the heated and pressurized paint is returned to the pump.An adjustable back pressure regulator is provided in the return line tomaintain the desired pressure at the spray v gun.

In many instances it is desirable to use a single pumping system tosupply two or more spray guns. In such systems where recirculation isrequired, it is known to provide a pump which supplies paint underpressure to a high pressure line. The high pressure line is connected toeach spray gun and when heated paint is required, a heater is usuallyprovided in the high pressure line upstream from the spray guns. Arecirculating or return line is connected to each gun and returns theexcess or recirculating paint to the pump. Examples of such systems areillustrated in the U.S. Pat. Nos. 2,022,481; 2,244,686; 2,266,354;3,023,968; 3,385,522; and 3,561,645.

Because such systems provide spray guns connected in parallel betweenthe high pressure supply line and the return line, it is necessary forthe supply and return lines to extend to each station at which a gun isprovided. Further, since theguns are in parallel, a malfunction in agiven portion of the system can cause flow to. be interrupted in suchportion. Still further, sufficient volume of pressurized paint must beprovided to insure the desired flow rates are maintained in each branchof the system.

In other instances, systems have been provided in which a'plurality ofguns are connected in series with all ofthe recirculating materialpassing from one gun to the next, less only the amount released in thespraying from each gun. Such a system is objectionable because of theaccumulated pressure drop which occurs, particularly in systems having arelatively large number of spray guns.

, SUMMARY OF THE INVENTION Located in the loop at each spray gun stationis a flow restriction and a pair of valve fittings with one on each sideof the associated flow restriction. The supply line of the associatedgun is connected to the fitting upstream of the flow restriction and thegun return line is connected to the associated fitting downstream of theflow restriction. Therefore, each gun is connected in parallel with anassociated flow restriction and in series with the other flowrestrictions and associated guns.

In one illustrated embodiment, the spray guns are airless and the flowrestrictions are adjustable pressure regulators which create a pressuredrop that is automatically maintained at an adjusted value. In practice,the pressure regulators are adjusted to just create sufficient pressuredrop to insure that the desired flow rate is maintained through thesupply and return lines which connect the associated guns to therecirculating loop in order to maintain material temperature at the guncomparable to main line temperature.

In another embodiment, air spray guns are utilized and a gun mountedpressure reducing regulator is provided at each gun to control thepressure of the paint supplied to the gun. v

Because only a single loop is required, the total amount of piping, whencompared to the parallel systems previously employed, is reduced in manyinstallations without encountering excessive pressure drops. Further,the recirculating flow is reliably maintained through all portions ofthe system, and it is not necessary to provide excessive quantities ofpressurized paint to maintain the desired flow rates in all portions ofthe system. I

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic illustration ofa preferred embodiment of this invention in which a single pump isconnected through a recirculating loop to three airless spray guns;

FIG. 2 is a fragmentary schematic illustration of the structural detailof the back pressure regulator and bleed system located at the intake ofthe pump;

FIG. 3 is a fragmentary schematic illustration of the structural detailof the automatic pressure regulator provided at each spray gun stationalong the recirculating loop;

FIG. 4 is a fragmentary schematic illustration of the structural detailof a second embodiment of back pressure regulator which maybe providedat the pump intake in place of the embodiment illustrated in FIG. 2;

and

FIG. 5 is an enlarged fragmentary view of an air spray gun with apressure reducing regulator which may be utilized in a system inaccordance with this invention.

The present invention is disclosed in an embodiment in which a singlerecirculating system supplies three separate paint spray devices withheated paint under pressure. However, it should be understood that thissystem is equally applicable for supplying other types of coatingmaterials and that the invention is not limited to painting per se orthe application of paint per se. Further, it should be understood thatthis system in accordance with this invention is suitable for supplyingsubstantially any number of devices and that the system incorporatingthree spray devices is only illustrated by way of an example on a systemincorporating this invention.

FIG. 1 illustrates this invention applied to an airless paint spraysystem. The system includes a pump provided with an inlet 11 and anoutlet 12. The illustrated pump is a reciprocating pump driven by acompressed air motor 13. Reference may be made to my US. Pat. No.3,0l8,968 for a more detailed description of such an air motor. Anysuitable pump may be used, but the illustrated pump is preferred sinceit is capable of utilizing relatively low pressure compressed air, whichis available in many industrial installations, to pressurize paint orthe like to relatively high pressures in the order of 500 to 3,000 psi.The particular pump illustrated is arranged so that the pressure andvolumetric output is substantially equal on both the extension andretraction strokes.

Connected to the output 12 is a pressure line 14 which supplies thepaint at the output pressure to a heater schematically illustrated at16. From the heater, a second line 17 supplies the paint under pressureto a first adjustable flow restriction 180. After passing through theflow restriction 18a, the paint still under pressure passes through apressure line 19 to a second flow restriction 18b which is similar instructure to the flow restriction 18a. Paint flowing past the secondflow restriction 18b flows through still another line 21 to a third flowrestriction 18c and thereafter through a line 22 and a back pressureregulator 23 to the pump inlet 11. The various pressure lines thereforeprovide a single recirculating loop through which paint flows from thepump outlet 12 back to the pump inlet 11 in which the three flowrestrictions 18a, 18b, and 180 are connected in series in therecirculating loop. The back pressure regulator 23 determines the exitpressure from the recirculating loop and, consequently, determines thepressure maintained in the recirculating loop.

The system also includes a supply line 24 which connects the pump 10 toa supply of paint schematically illustrated by the container 26. Inaddition, a bleed line 27 and bleed valve 28 are provided for bleedingthe system to eliminate air therein and to facilitate flushing of thesystem.

Three similar airless spray guns 29a, 29b, and 29c are respectivelyconnected in the system adjacent the associated flow restrictions 18athrough 180. Each spray gun is provided with a supply line 31 whichconnects through a shutoff valve 32 to the recirculating loopimmediately upstream from the associated flow restriction. A return line33 connects each spray gun through a shutoff valve 34 to the loopimmediately downstream from the associated flow restriction.Consequently, each spray device is connected in parallel with anassociated flow restriction and each spray gun and associated flowrestriction assembly is connected in series with the other assemblies inthe recirculating loop.

FIG. 2 illustrates the structural detail of the back pressure regulator22 and its associated apparatus. Return flow passing through the line 23enters a fourway fitting 36 and is connected through a fitting 37 to thebody 38 of the back pressure regulator 23. A pressure gage 39 is alsoconnected to the fitting 36 to provide a visual indication of the backpressure maintained by the back pressure regulator 23. The bleed valve28 also connects with the fitting 36 and in normal operation, it ismaintained closed as illustrated. The function of the bleed valve 28 isdiscussed in detail below.

The inner end of the fitting 37 is provided with a valve seat 41 adaptedto be engaged by a ball valve 42 carried by a valve stem 43. The valvestem is inturn carried by an adjusting stem 44 which is threaded into agland nut 46 mounted on the body 38. A spring 47 extends between athrust member 48 and the valve stem 43 providing a force whichresiliently urges the ball valve 42 toward the seat 41. The value of thespring force is adjusted by threading the adjusting stem 44 eitherinward or outward with respect to the body 38.

When the pressure of the returning fluid upstream of the valve seat 41reaches a pressure determined by the force of the spring 47, itovercomes the action of the spring and causes the valve element 42 tolift away from the seat 41 and permits return flow to the inlet 11 ofthe pump. On the other hand as flow occurs and pressure drops, thespring overcomes the pressure-induced forces and moves the valve towardthe seat to throttle the flow. Consequently, the regulator automaticallyfunctions to maintain a return pressure, the value of which isdetermined by the adjusted force of the spring 47. When the spray gunsare operated, the recirculating flow is not sufficient to supply thepump and makeup flow is automatically drawn through the line 24 in thepump inlet 11.

FIG. 3 illustrates the structural detail of the flow restrictions 18. Itshould be understood that each of the flow restrictions 18a through 18cis provided with the structure illustrated in FIG. 3. The inlet linethrough which the paint under pressure is supplied to the flowrestriction is connected through a fitting 51 to a tee 52. The shutoffvalve 32 is also connected to the tee 52 through a fitting 53. The thirdport of the tee 52 is connected to a fitting 54 provided with a valveseat 56 at its inner end. In the illustrated embodiment, the fitting 54forms part of a pressure regulator 57 which includes a spring 58 urginga ball valve 59 toward the seat with a force having a value determinedby the adjustment of the regulator stem 60. This regulator operates toautomatically produce a relatively low pressure drop as the paint flowstherethrough but must be able to withstand the relative high pressureexisting in the recirculating loop.

The shutoff valve 32 connects with the supply line 31 to the associatedgun, and the return line 33 connects through the shutoff valve 34 to theregulator downstream from the ball valve 59. The flow passing throughthe valve seat 56 along with the return flow through the line 33 passesout of the assembly through the downstream line of the recirculatingloop which is connected through a fitting 65.

In operation, the pump 10 pressurizes the paint and pumps it through itsoutlet 12 to the recirculating loop. After the paint passes through theheater 16, it reaches the first flow restriction 18a. The flow then isdivided with most of the flow passing directly through the automaticpressure regulator 57 and the other part of the flow passing through thesupply line 31 to the spray gun 29a and then back to the loop throughthe return line 33. During operation, the valves 32 and 34 are open topermit flow to the gun. The pressure regulator 57 of the flowrestriction 18a is adjusted to provide a relatively low pressure dropusually in the order of 3 to 5 psi. This pressure drop is adjusted to besufficient to cause the required recirculating flow through the supplyline to the gun 29a and back through the return line 33 from the gun.Preferably, the regulator 57 is adjusted to the lowest possible pressuredrop which will produce the required flow rate through the circuitconnecting the gun 29a.

When the lines 31 and 33 are long or smaller in diameter or when theviscosity of the paint being pumped is high, the regulator 57 isadjusted to provide a higher pressure drop to insure proper flow to thegun. However, excessive pressure drops are not desirable since the powerloss resulting from the flow through the regulator increases with higherpressure drops.

From the first spray gun 29a and flow restriction 18a, the paint underpressure continues through the line 19 to the second spray gun 29b andflow restriction 18b. Here again, the flow is divided with sufficientflow passing through the supply and return lines 31 and 33 to insureproper recirculating flow to the gun 29b. The remaining flow passesdirectly through the regulator 57 of the second flow restriction 18b.

The flow then continues through the line 21 of the third flowrestriction 180 wherein the flow is again divided with a portion of theflow passing through the supply and return lines 31 and 33 to the gun29c and the remaining flow passing directly through the regulator 57 ofthe third flow restriction 180.

The recirculating flow then passes through the line 22 and back to theinlet 11 through the back pressure regulator 23. The back pressureregulator 23 is adjusted to automatically maintain the desiredrecirculating pressure in the system. For example, the back pressureregulator is usually adjusted to maintain a back pressure in the orderof 500 to 3,000 pounds when the system is used for operation of airlessspray guns. Because the pressure drop occurring at each of the flowrestrictions is relatively small, the operating pressure available ateach spray gun is fully adequate, and the difference in pressure betweenthe upstream guns and the downstream guns is insignificant.

The shutoff valves 32 and 34 are provided to permit the removal of theconnected supply and return lines from a particular operating stationwithout requiring an interruption in the operation of the system. Whensuch removal is required, the two associated valves 31 and 34 are closedand the full flow in the system occurs through the associated pressureregulator 57. If such removal is likely to continue for a substantialperiod of time while the system continues to operate, it is preferableto adjust the associated pressure regulator to provide a minimumpressure drop so that the power loss created by flow through such flowrestriction will be minimized.

Although automatic pressure regulators of the adjustable typeillustrated are preferred at each of the painting stations, in someinstances where such adjustment is not required a fixed orifice may beprovided. In such a system, however, adjustability and automaticresponse is not provided.

The bleed valve and connected bleed line 27 are normally used onlyduring the flushing of the system and during the start-up of the system.Flushing of the system is usually accomplished by connecting the line 24to a source of solvent or the like while connecting the line 27 toreturn the solvent to such source. During the flushing operation, thebleed valve 28 is opened to allow a solvent to be drawn into the systemfor circulation therethrough followed by discharge of the solventthrough the bleed valve 28 and bleed line 27.

The elimination of air from the system is accomplished in a similarmanner in that the bleed valve 28 remains open during the initialoperation of the pump to allow air to be bled out of the system. Duringsuch bleeding operation, it is customary to adjust the back pressureregulator 23 to provide a sufficient pressure drop to insure that theflow occurs out through the bleed valve 28. After bleeding is completed,the bleed valve 28 is closed and the recirculating flow occurs in themanner described above.

FIG. 4 illustrates a second back pressure regulator structure which maybe substituted for the regulator 23 where desired. The back pressureregulator illustrated in FIG. 4 is disclosed and claimed in my U.S. Pat.No. 3,388,866 referred to above, and reference may be made to thatpatent for a more detailed description of the structure and mode ofoperation of such a regulator. When the regulator of the typeillustrated in FIG. 4 is utilized, the return line 22 is connectedthrough a tee fitting 6l'to the inlet 62 of the back pressure valve 63.A pressure gage 64 is mounted on the tee 61 to provide a visualindication of the back pressure present in the system. In thisembodiment, the bleed valve 28 is connected to the valve 63 in a mannerso that it is in direct communication with the return line 22. Apressure regulating assembly 64 is provided to adjustably regulate theback pressure in the line 22. Here again, a spring-biased ball valve 66is resiliently urged toward engagement with a valve seat 67 with a forcewhich is adjustable. This valve automatically operates to maintain aback pressure at a value of which is determined by the adjustment of theregulator 63. Downstream of the regulator 63, a second pressure reducingvalve assembly 67 is provided. This regulating valve or assembly is notadjustable, but provides a ball valve 68 which is urged by a spring 69toward a valve seat 71 with a constant spring force. Preferably, thevalve assembly 67 is constructed so that it provides a minimum backpressure in the order of 10 to 20 pounds on the pressure regulator 63.Such minimum back pressure maintains a load on the seal 72 and functionsto prevent leakage and induction of air into the system as described inmy U.S. Pat. No. 3,388,866. The downstream side of the valve assembly 67is connected to the inlet 1 l of the pump 10 along with the supply line24.

This embodiment of a back pressure regulator has the advantage ofproviding greater reliability in preventing induction of air into thesystem and is particularly desirable in combination with the presentsystem since the present system involves substantially greater volume inthe recirculating loop and, therefore, greater bleeding problems thansmall systems employing a single gun and a small recirculating path.

Preferably, the lines l4, l7, 19, 21, and 22 are sized to provide arelatively low pressure drop during flow therethrough. The supply andreturn lines 31 and 33 connecting the loop of each of the guns, however,may be smaller since all of the flow does not pass through such lines.Of course, smaller lines are desirable since they are easier to handle.For example, in systems in which the recirculating loop is 350 feet longor less and the liquid being sprayed has a viscosity of not more than100 seconds Zahn 2, it is desirable to form the recirculating loop oflines having an inside diameter of at least one-half inch. Because theflow through the lines 31 and 33 is not the full flow through thesystem, smaller diameter lines can and are preferably used for suchportions of the system. When greater length is present in therecirculating loop or greater flow rates are required due to thepresence of larger numbers of spray guns, the recirculating loop shouldbe formed of larger diameter lines. Also, when higher viscositymaterials are being handled, larger line diameters are required.

The present invention is also applicable to conventional air spray guns.In such a system, air spray guns are connected to the system insubstantially the same manner as illustrated in FIG. 1. In air spraysystems, the recirculating loop is normally maintained at a much lowerpressure than in an airless spray system. In many instances, the workingpressure of the paint supplied to an air spray gun is in the order of 75psi. In such a system, the exit pressure from the recirculating loop isadjusted to maintain the required working pressure at the last gun inthe loop. The upstream guns, because of the pressure drop at each spraystation, therefore, are supplied with paint at higher pressures thanrequired. If the system utilizes the relatively small number of guns sothat the accumulated pressure drop through the recirculating loop isinsufficient to produce problems at the upstream guns, a pressurereducing regulator is not provided. The guns are connected in the systemin the manner illustrated in FIG. 1.

In instances where the accumulated pressure drop in the recirculatingloop is such that is produces excessive pressure at the upstream guns,for example in systems employing a large number of guns in the singlerecirculating loop, a gun mounted pressure reducing valve may beprovided to automatically reduce the pressure supplied to the guns tothe desired value. FIG. illustrates an air spray gun 81 with such gunmounted regulator. The air spray gun is connected to the supply andreturn lines 31 and 33 through a regulator 82 and is separately suppliedwith compressed air through an air line 83. The regulator includes ahousing assembly 84 having an inlet 86 connected to the two lines 31 and33. The gun 81 is connected through an outlet passage 87 separated fromthe inlet 86 by a valve assembly 88. The valve assembly 88 includes avalve seat member 89 and a valve poppet 91 supported by a flexiblediaphragm 92. A spring 93 urges the valve poppet toward the closedposition illustrated, and an adjustable spring 94 urges the poppet valvetoward an open position with a force which is adjusted by a valve stem96.

This regulator operates automatically to maintain the outlet pressure tothe gun at a pressure determined by the setting of the valve stem 96.When the outlet pressure drops below the required pressure, the force ofthe paint on the diaphragm 92 decreases allowing the spring 94 to openthe valve. 0n the other hand, when the pressure in the outlet builds upto the regulating pressure, the force of the paint on the diaphragmcauses the diaphragm to move the valve toward the seated position. Theregulator therefore functions to reduce the pressure of the paintsupplied to the gun from the pressure at the adjacent portion of therecirculating loop to the working pressure required by the gun.

In a recirculating system of the type illustrated generally in FIG. 1 inwhich air spray guns of the type illustrated in FIG. 5 are installed,the recirculating pressure is adjusted to supply at least the requiredworking pressure at the downstream gun, for example psi. In such asystem, the pressure at an upstream gun is higher due to the pressuredrops occurring at the various units 18 and due to the pressure dropresulting from the resistance to flow provided by the lines.Consequently, the pressure at the upstream guns may be substantiallyhigher than 75 psi and may be, for example, in the order of psi or morewhen the system is provided with a large number of guns. In such asystem, the pressure reducing regulator 82 is adjusted to reduce thepressure supplied to the upstream guns to a pressure in the order of 75psi.

Although preferred embodiments of this invention are illustrated, it isto be understood the various modifications and rearrangements of partsmay be resorted to without departing from the scope of the inventiondisclosed and claimed herein.

I claim:

1. A liquid distribution system comprising pump means having an inletand an outlet, a recirculating loop connected at one end to said outletand at its other end to said inlet, said pump means operating torecirculate liquid under pressure through said loop, at least twostations each provided with a flow restriction connected in the loop inseries with each other, at least two liquid dispersing devices eachprovided with a supply line and a return line, and means for connectingthe supply line of each liquid dispersing device to said loop at theassociated station upstream from its associated flow restriction and forconnecting the associated return line to said loop at the associatedstation downstream from its associated flow restriction, each flowrestriction being operable to permit flow therethrough while providingsufficient pressure drop between the associated supply and return linesto cause a flow through such lines.

2. A liquid distribution system as set forth in claim 1 wherein backpressure means are provided in said loop downstream from said stations.

3. A liquid distribution system as set forth in claim 2 wherein saidback pressure means is an adjustable automatic regulating valve.

4. A liquid distribution system as set forth in claim 3 wherein saidback pressure means is adjusted to maintain a pressure having a valuewhich is many times the value of the pressure drop produced by said flowrestrictions.

5. A liquid distribution system as set forth in claim 4 wherein saidliquid dispersing devices are airless spray guns.

6. A liquid distribution system as set forth in claim 5 wherein eachflow restriction is adjustable.

7. A liquid distribution system as set forth in claim 6 wherein eachflow restriction is an automatic pressure regulating valve.

8. A liquid distribution system as set forth in claim 7 wherein saidback pressure means includes two pressure reducing valves connected inseries only one of which is adjustable.

9. A liquid distribution system as set forth in claim 2 wherein eachflow restriction is an automatic pressure regulating valve.

10. A liquid distribution system as set forth in claim 9 wherein ashutoff valve is provided in each supply and return line allowing suchlines to be removed while said loop is pressurized.

11. A liquid distribution system as set forth in claim 10 wherein aheater is provided in said loop upstream from said stations.

12. A liquid distribution system as set forth in claim 9 wherein saidloop is formed of first conduit means and said supply and return linesare formed of second conduit means, said second conduit means having asubstantially smaller internal cross sectional area than said firstconduit means.

13. A liquid distribution system comprising a recirculating loop, pumpmeans connected to pressurize liquid in said loop and recirculate suchliquid around said loop, a plurality of liquid dispersing devicesconnected to said loop in series with each other, and a flow restrictionconnected in parallel with each dispersing device operable to cause aportion of the liquid flowing around said loop to bypass the associateddispersing device.

14. A liquid distribution system as set forth in claim 13 wherein eachflow restriction is an adjustable automatic pressure regulating valve.

15. A liquid distribution system as set forth in claim 14 wherein saiddispersing devices are airless spray guns.

1. A liquid distribution system comprising pump means having an inletand an outlet, a recirculating loop connected at one end to said outletand at its other end to said inlet, said pump means operating torecirculate liquid under pressure through said loop, at least twostations each provided with a flow restriction connected in the loop inseries with each other, at least two liquid dispersing devices eachprovided with a supply line and a return line, and means for connectingthe supply line of each liquid dispersing device to said loop at theassociated station upstream from its associated flow restriction and forconnecting the associated return line to said loop at the associatedstation downstream from its associated flow restriction, each flowrestriction being operable to permit flow therethrough while providingsufficient pressure drop between the associated supply and return linesto cause a flow through such lines.
 1. A liquid distribution systemcomprising pump means having an inlet and an outlet, a recirculatingloop connected at one end to said outlet and at its other end to saidinlet, said pump means operating to recirculate liquid under pressurethrough said loop, at least two stations each provided with a flowrestriction connected in the loop in series with each other, at leasttwo liquid dispersing devices each provided with a supply line and areturn line, and means for connecting the supply line of each liquiddispersing device to said loop at the associated station upstream fromits associated flow restriction and for connecting the associated returnline to said loop at the associated station downstream from itsassociated flow restriction, each flow restriction being operable topermit flow therethrough while providing sufficient pressure dropbetween the associated supply and return lines to cause a flow throughsuch lines.
 2. A liquid distribution system as set forth in claim 1wherein back pressure means are provided in said loop downstream fromsaid stations.
 3. A liquid distribution system as set forth in claim 2wherein said back pressure means is an adjustable automatic regulatingvalve.
 4. A liquid distribution system as set forth in claim 3 whereinsaid back pressure means is adjusted to maintain a pressure having avalue which is many times the value of the pressure drop produced bysaId flow restrictions.
 5. A liquid distribution system as set forth inclaim 4 wherein said liquid dispersing devices are airless spray guns.6. A liquid distribution system as set forth in claim 5 wherein eachflow restriction is adjustable.
 7. A liquid distribution system as setforth in claim 6 wherein each flow restriction is an automatic pressureregulating valve.
 8. A liquid distribution system as set forth in claim7 wherein said back pressure means includes two pressure reducing valvesconnected in series only one of which is adjustable.
 9. A liquiddistribution system as set forth in claim 2 wherein each flowrestriction is an automatic pressure regulating valve.
 10. A liquiddistribution system as set forth in claim 9 wherein a shutoff valve isprovided in each supply and return line allowing such lines to beremoved while said loop is pressurized.
 11. A liquid distribution systemas set forth in claim 10 wherein a heater is provided in said loopupstream from said stations.
 12. A liquid distribution system as setforth in claim 9 wherein said loop is formed of first conduit means andsaid supply and return lines are formed of second conduit means, saidsecond conduit means having a substantially smaller internal crosssectional area than said first conduit means.
 13. A liquid distributionsystem comprising a recirculating loop, pump means connected topressurize liquid in said loop and recirculate such liquid around saidloop, a plurality of liquid dispersing devices connected to said loop inseries with each other, and a flow restriction connected in parallelwith each dispersing device operable to cause a portion of the liquidflowing around said loop to bypass the associated dispersing device. 14.A liquid distribution system as set forth in claim 13 wherein each flowrestriction is an adjustable automatic pressure regulating valve.